Non-volatile 2D MoS2/black phosphorus heterojunction photodiodes in the near- to mid-infrared region

Cutting-edge mid-wavelength infrared (MWIR) sensing technologies leverage infrared photodetectors, memory units, and computing units to enhance machine vision. Real-time processing and decision-making challenges emerge with the increasing number of intelligent pixels. However, current operations are limited to in-sensor computing capabilities for near-infrared technology, and high-performance MWIR detectors for multi-state switching functions are lacking. Here, we demonstrate a non-volatile MoS2/black phosphorus (BP) heterojunction MWIR photovoltaic detector featuring a semi-floating gate structure design, integrating near- to mid-infrared photodetection, memory and computing (PMC) functionalities. The PMC device exhibits the property of being able to store a stable responsivity, which varies linearly with the stored conductance state. Significantly, device weights (stable responsivity) can be programmed with power consumption as low as 1.8 fJ, and the blackbody peak responsivity can reach 1.68 A/W for the MWIR band. In the simulation of Faster Region with convolution neural network (CNN) based on the FLIR dataset, the PMC hardware responsivity weights can reach 89% mean Average Precision index of the feature extraction network software weights. This MWIR photovoltaic detector, with its versatile functionalities, holds significant promise for applications in advanced infrared object detection and recognition systems.

The scale bar is 10 μm.d, The conductance state can be regulated by the 1550 nm laser pulse.A maximum photoresponsivity of 0.0125 A/W is obtained at 2.4 mW mm −2 .The BP flash memory device shows characteristics of a volatile synapse.Challenges arise in achieving both electrical modulation and optical absorption due to the need for thin materials for electrical modulation and thick materials for optical absorption.Therefore, this necessitates using thin materials for conductive state changes and thick materials for optical absorption.

Pulse modulation effect
The electric field in h-BN and MoS2 layers is around 14 MV cm -1 when a 30 V or -30 V voltage is applied, which is much larger than the 7.43 MV cm -1 that can induce the FN tunneling effect in the Cr/h-BN/graphene heterostructure.So, the FN tunneling current of carriers should be large in the PMC device and play an important role in the short writing time.Continuing to increase the gate voltage theoretically leads to higher tunnelling currents and program time can be as low as 1 ns 1 .However, the minimum pulse width of the instrument (B1500 SPGU) limits further exploration (Supplementary Fig. 12).
Different pulse settings tend to have a direct effect on the electrical dynamic range of the device, the number of states stored in the device and the average state interval.
Here we tested the modulation of some pulses in Supplementary Fig. 13.By analyzing the test results, the following main conclusions were summarized in Supplementary Table 1.(1) For pulses of the same time width, increasing pulse amplitude results in a larger conductance modulation range and a smaller number of conductance states.(2)   For pulses of the same amplitude, increasing pulse width leads to a larger conductance modulation range and a smaller number of conductance states.In the table, we quantitatively show the range of conductance states, the number of conductance states and the average conductance interval corresponding to the pulse modulation.These findings contribute to a comprehensive understanding of the impact of pulse parameters on device performance.

Retention time mechanism
The loss of the conductance state is mainly due to the leakage of the captured charge by tunneling under weak fields, which is thus manifested by a change in the threshold voltage.Here we define the conductance state failure criterion as a 50% loss of captured charge, in other words, the time corresponding to the shift of the threshold voltage difference to 50% of the initial value is the conductance retention time.In the In consideration of the susceptibility of BP material to oxidation, we coated a layer of PMMA on the surface of the device to insulate it from moisture and oxygen.We show the original memory characteristics of PMC devices in Supplementary Fig. 16.
As a comparison, we show the memory characteristics of PMC devices after 15 days in Supplementary Fig. 17.The results show that the PMMA coating can effectively reduce the effect of BP oxidation on the device performance.
In the near-infrared region, the photon energy is sufficiently high to directly excite electron-hole pairs, generating a photocurrent.Therefore, the photovoltaic effect predominantly governs this region.This direct photo-generated carrier generation process is linear.In the mid-wave infrared region, the photon energy is lower and more susceptible to thermal effects, often producing photo-thermoelectric and photobolometric effects [2][3][4] .When photons are absorbed and converted into thermal energy, a local or overall temperature increase occurs in the material, leading to changes in the photocurrent due to temperature variation.This process is influenced by the duration of illumination, introducing non-linear characteristics.The solution is to get the mapping function by using a fitting method.In Supplementary Fig. 26, The red dots are the data from the test, and the red line is an exponential fit to the test data.R 2 = 0.99013, 0.94944, 0.98469 and 0.98597, respectively.

Calculations of blackbody detection
The total incident power of blackbody radiation on the device surface can be calculated using the formula 5 : ( Where α is the modulation factor, ε is the average emissivity of the blackbody radiation source, σ is the Stefan-Boltzmann constant.T is the blackbody radiation source temperature, T0 is the test environment temperature, A is the blackbody radiation source area, An is the device area and L is the distance of the photodetector from the aperture. The equation of responsivity ( ) under blackbody radiation is: ( where Iph is blackbody photocurrent. The response spectrum obtained by FTIR is only a relative response spectrum.The response spectrum measured by FTIR is an equal power curve in which the radiated power at any wavelength is equal.After calibration of the blackbody response, the responsivity of the response spectrum of the photodetector can be obtained.The relative responsivity spectrum R'(λ) is obtained from the FTIR characterization: (3) Since the blackbody radiation has a continuous spectrum and the emissivity of each wavelength is different, the blackbody light signal generated by the photodetector is the sum of the signals generated by the radiation at each wavelength: The ratio of the blackbody responsivity (Rblackbody) to the peak responsivity (R(λp)) of an infrared photodetector is a constant, the g-factor.The g-factor can be calculated from the tested R'(λ) and the blackbody spectral emissivity (()).Therefore, by calculating the g-factor, the peak responsivity (R(λp)), quantum efficiency (QE(λp)) and the peak detectivity (D*(λp)) can be obtained: or where h is Planck's constant, c is the speed of light in vacuum, λ is the photon wavelength, is the electrical bandwidth.
The edge of an image represents its fundamental feature, where "edge" refers to the local characteristic discontinuity in the image.Mutations in grayscale or structural information are identified as edges, including variations in gray level, the mutation of color, the mutation of texture structure and so on.An edge makes the transition from one region and to another, and leveraging this characteristic facilitates image segmentation.Edge detection is usually carried out by Sobel, Scharr and Laplacian operators, which can be replaced by device weights 7 .
symbol represents the probability of the corresponding GT(ground truth) prediction.Lreg is the loss of rpn_loss_anchor layer, Lcls is the loss of rpn_classification_loss layer.Ncls is the number of network trainings used for anchor classification, Nreg is the number of network trainings used for bounding box regression.
The concept of IOU is a measure of how much the predicted frame overlaps with the true frame, which is meant to elicit the other two indicators: precision and recall. ( Where SIntersection represents the overlapping area between the predicted and actual boxes, SSum corresponds to the total area occupied by both the prediction and actual boxes.TP (True Positives) means that they are divided into positive samples and they are divided correctly.TN (True Negatives) means that the sample was divided into negative samples, and it was divided correctly.FP (False Positives) means that the sample was divided into positive samples, but was divided into wrong samples (in fact, the sample was negative).FN (False Negatives) means that the sample was divided into negative samples, but the division was wrong (in fact, the sample was positive).
Furthermore, there are two important concepts: * Take the processing of a 1MB image as an example

Supplementary Fig. 8 |
Electrical characteristics and infrared laser regulation of BP/h-BN/graphene flash memory.a, BP/h-BN/graphene flash memory structure controlled by a back gate.b, Transfer characteristic curve with memory windows.The dynamic range of electrical regulation is over 10 4 .c, Applying an electrical pulse set of (VBG, pulse= -30 V for 20 ns, 1 Hz) regulates the conductance state from 1 pA to 350 nA.Inset: top view of the optical microscope photograph of BP flash memory device.
Reconfigurable characteristics and endurance testing.a, reconfiguration features between 1 pA and 1 µA dynamic ranges (20 ns pulse width).b, Device endurance test.The -30 V/20 ns is used to program the device to high resistance state (HRS), and the 30 V/1s is used to program the device to a low resistance state (LRS).The channel current is read out at VBG = 0 V, VDS = 1 V.The PMC device maintains stable operation for up to 10 4 pulse cycles, with the low conductivity state failing at 2 nA after 10 5 pulse cycles.

Fig. 3a of
Fig.3aof the previous manuscript, we presented the retention time of approximately

Table 2 .
AP) in fact refers to the area under the curve drawn using different combinations of Precision and Recall's points.By considering different confidence levels, diverse Precisions and Recalls are obtained.With sufficiently dense confidence levels, numerous Precisions and Recalls can be acquired.. Precision and Recall can form a curve, and the area beneath this curve represents the AP value for a specific class.The mean Average Precision (mAP) is the average of the AP values across all classes, providing a comprehensively evaluation of recognition results.Response weight sequence (A/W).

Table 3
Trained weight.

Table 4
PMC device weight.